Avalanche photodiodes are semiconductor-based photodetectors where light incident on a reverse biased semiconductor junction changes the current flowing through the junction. The current flow can be used to indicate the incident light intensity.
When used in fibre-optic communication systems, avalanche photodiodes typically require a reverse bias voltage of between 10 and 100 V. To provide electrical decoupling from the effects of the incident signal's optical modulation, a bypass capacitor is typically placed across the avalanche photodiode. It has been noted that sudden increases in incident light level, as might occur when using an Optical Time Domain Reflectometer (OTDR) to determine the location of a fibre break, can lead to a corresponding increase in the current flowing through the avalanche photodiode to the extent that the crystal structure of the avalanche photodiode is damaged.